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Choosing angle sensors for power steering, motors and robotics

01 September 2020

As factories and vehicles become more automated, accurate and low-latency sensing of motor shaft speed and position is critical for process control, system reliability, and safety. To address these needs, designers require angular rotation sensors that are fast and precise, with the flexibility to address magnetic field variations and axial misalignment.

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Complicating the issue for designers are ever-present cost and time pressures. Furthermore, the nature of the operating environments for industrial and automotive applications can be challenging in terms of chemicals and oils, as well as temperatures and EMI. 

The role of angle sensors 

Angle sensors are used to sense motor shaft position, speed variations for steering angle sensing for automobiles, and high-precision control in robotic systems. They determine the absolute angular position of a diametrically magnetised cylinder on a rotating shaft by detecting the orientation of an applied magnetic field and measuring its sine and cosine components. As the shaft may be rotating at high speed, the data from the sensor must be acquired and processed quickly, with minimal latency.

One of four magnetic technologies – Hall effect, anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) (Figure 1) – is typically used. When using any of these technologies, designers must first determine a suitable distance from the magnet surface to the sensor based on specific parameters, such as magnetic properties, sensor specification and assembly tolerances.

This air gap must be consistent with parameters such as magnet size and remanence, also known as residual magnetisation. Designers must also ensure that air gap variations don’t result in magnetic fields that are either too low or too high. This requires careful consideration of the appropriate magnet for the application’s air gap (Figure 2).

That said, angle sensors can support a wide range of spatial configurations and magnetic field strengths, including both off-axis or side-shaft mounting and end-of-shaft configurations. To help accommodate variations, on-chip non-volatile memory is used to store configuration parameters such as reference zero angle position, ABZ encoder settings, and phase information for the motor windings.

Next, the device’s ability to detect various magnetic field strengths allows developers to customise the angle sensor for specific functions like diagnostics and axial movement sensing. The availability of programmable magnetic field strength thresholds also facilitates the implementation of a push or pull button function outputted as two logic signals.

However, while features like speed, low latency and resolution depend on application requirements, safety is at the heart of angle sensor designs. 

Read the full article in the September issue of DPA.

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